This invention relates generally to a liquid metal nuclear reactor fuel pin, and more particularly to a fuel pin having an improved metallic fuel arrangement.
Conventional metallic fuel pin assemblies typically include pressure cast fuel alloys which are loaded into cylindrical cladding tubes sealed at their ends. Mechanical interaction during fission between the fuel and cladding is prevented by a large radial gap. Typically this gap takes up about 25% of the volume within the cladding tube. Also included in the pin is a fission gas plenum in the form of a chamber which receives fission gases released by the fuel during operation of the reactor.
Because of the high thermal conductivity of metallic fuels (20 W/m-k) and their relatively low melting points (approximately 2200.degree. F.), the existence of the large gas gaps, without any coolant bond, could cause excessive fuel temperature and perhaps even fuel melt. This is undesirable because it would redistribute the fuel to the bottom of the fuel column, which would disturb the porosity balance and render the location of the fuel material somewhat unpredictable.
To preclude fuel melting, prior designs have provided a sodium thermal bond between the fission material and the cladding. The high thermal conductivity of the sodium, compared to gas, keeps fuel temperatures acceptable. During operation, metallic fuel swelling closes the gap between fuel and cladding, and this displaces the sodium away from the fuel and into the fission gas plenum. This displacement reduces the available fission gas plenum volume, which diminishes the capacity of the system, or requires additional fuel pin length. Any such increase in length dramatically increases reactor containment costs.
Another limitation of conventional fuel pin designs is that the use of a sodium thermal bond normally requires that the fission gas plenum be disposed above the fuel, in an upper part of the fuel pin. This leads to inefficient utilization of fission gas plenum space because the reactor coolant system circulates coolant upwardly between the various fuel pins. Thus, system coolant passes the fuel region before it passes the plenum. The resulting heating of the coolant prior to contact with the plenum increases the fission gas plenum pressure and temperature, or requires additional fuel pin length. This length increase again dramatically increases reactor containment costs.
There are other drawbacks with the use of sodium or another coolant as a thermal bond between the fuel and the cladding. Because of the proximity of the coolant to the metallic fuel, it is an additional contaminated waste product which must be dealt with in reprocessing the fuel elements. Clean up and/or storage of such waste products is very expensive and troublesome. Finally, the requirement of a fuel pin thermal bond also increases the expense of the system.
Another disadvantage with existing metallic fuel systems is that there is typically far more power being generated from the areas near the axial center of the fuel stack than is being generated near the ends of the fuel stack. This is because the neutron concentration is greater near the axial center of the fuel stack than at the ends of the fuel stack where neutron leakage occurs. It has long been thought that if there was a way to increase the power output adjacent the ends of the fuel stack, then a dramatic increase in power output could be realized.
Yet another typical feature of existing systems is a tag gas capsule which contains a tag gas comprising specific mixtures of different isotopes of gases that can readily be sensed from a position outside of the reactor core. Thus, if there is leakage from one or more of the fuel pins, such leakage can readily be determined. It is also possible to use different gas mixtures for each fuel assembly position so that the tag gas sensor can determine in which fuel assembly position there is a leak as well as the presence of a leak.
In one embodiment of the present invention, there is disclosed a fuel pin design which includes a temperature sensitive tag gas capsule that releases tag gas into the fuel pin during reactor operation. By releasing tag gas into the fuel pin during reactor operation, the power-to-melt characteristics during reactor start up is improved.
Another embodiment of the invention discloses a fuel design which permits the elimination of the tag gas capsule while still providing pin leakage sensing capability. Eliminating of the tag gas capsule reduces the cost of the reactor both by eliminating an additional component and by decreasing the necessary length of the fuel pin.
It is an object of the present invention to avoid the drawbacks and limitations in the prior art. More specifically, the invention has as objects the following: (1) to develop a fuel system for a nuclear reactor which does not require a thermal bond between the fuel material and the cladding; (2) the provision of a fuel pin design which increases the effective amount of plenum space to receive fission gases, without increasing the size of the fuel pin; (3) to provide a nuclear power plant which generates less contaminated waste than conventional designs; (4) to provide a fuel pin design which results in more effective transfer of heat from the fuel pin to the reactor coolant system; (5) to develop a fuel pin which effectively increases the amount of power generated adjacent the ends of the fuel stack; (6) to provide a tag gas capsule which is temperature sensitive and releases tag gas within the fuel pin during reactor operation; (7) the provision of a fuel design which permits the elimination of the tag gas capsule, while still providing pin leakage sensing capability; and (8) to provide various fuel system alternatives which maximize reactor output while permitting a simplification of plant design.
The present invention achieves the above objects by providing various means for adding porosity to the fuel stack. Another way to describe the invention is to provide means for decreasing the effective smear density of the fuel. Such means could take the form of one or more axially extending channels extending through the fuel. They could take the form of spaced, axially extending peripheral flutes. Another alternative would be to provide small radial gaps or voids within the stack, or to form fuel spheroids which would provide the desired porosity.
Because the invention permits elimination of the annular fuel-to-cladding gap, radial transfer of heat from the pin to the reactor coolant system is enhanced, thus preventing fuel melt and resulting axial relocation. Expansion of the fuel is facilitated by the axial channels or the initial porosity of the fuel; that is, the channels or gaps in the fuel merely fill with the expanding fuel as fission is taking place.
A particular advantage of the use of spheroids or some other configuration which exhibits internal radial porosity is that means can be provided for exerting pressure on the end regions of the fuel stack. This pressure does not typically pass through the entire stack, so that the end regions are compacted more than the center region, thus increasing the amount of fuel at those portions. This permits a flattening of the axial fuel pin power distribution curve which can result in significant increases in reactor power output without any meaningful increase in assembly or operating expense.
Another advantage of the porous fuel stack of the present invention is that tag gas can be injected directly into the fuel region of the pin. Thus, as the fuel swells during fission, that gas is passed into the fission gas plenum. If there is leakage in the pin, the tag gas will pass out of the pin, just as though a separate tag gas capsule was provided.
Other objects, features and advantages of the present invention will become apparent upon reading the following detailed description in conjunction with the accompanying drawings.